LTE (Long Term Evolution), which is a succeeding communication scheme to WCDMA (Wideband Code Division Multiple Access), HSDPA (High Speed Downlink Packet Access) and HSUPA (High Speed Uplink Packet Access), is defined in WCDMA standard organization 3GPP (Release-8). As a radio access scheme in Release-8 LTE (hereinafter referred to as “REL8-LTE”), there is PFDMA (Orthogonal Frequency Division Multiplexing Access) defined for downlink.
The OFDM scheme is a multi-carrier transmission scheme in which a frequency band is divided into a plurality of narrow frequency bands (subcarriers) and data is transmitted in each of the subcarriers. As the subcarriers are arranged densely in the frequency axis while orthogonalized, it is possible to realize higher-speed transmission and increase the frequency use efficiency.
Further, the REL8-LTE defines a downlink reference signal structure. The downlink reference signal is used for the purpose of 1) measuring a downlink CQI (Channel Quality Indicator) for scheduling and adaptive control, 2) channel estimation for downlink synchronous detection in a user terminal (hereinafter referred to as LTE terminal) that supports the REL8-LTE and 3) estimating a state of downlink transmission channel for cell search and handover.
Furthermore, the REL8-LTE defines a radio transmission method, MIMO (Multiple-Input Multiple-Output), for improving the communication quality by providing a plurality of antennas in each of a transmitter and a receiver (see NPL1). This MIMO is classified into single MIMO in which the transmission layers (transmission information sequences) to be transmitted simultaneously are all of one user and multiuser MIMO in which they are of different users.
In the single user MIMO, four transmission antennas at the maximum are used to spatially multiplex four layers at the base station. Each layer does not correspond one-to-one with a transmission antenna, but is transmission via all transmission antennas using different transmission phase/amplitude control (precoding). With precoding, layers ideally transmitted simultaneously are received as orthogonalized at the receiver (they do not interfere with each other). As such, a precoding vector (weighting of each transmission antenna) is determined in such a manner that transmission layers (transmission information sequences) to be transmitted simultaneously do not interfere with each other and in consideration of fading so as to receive with higher SINR at an LTE terminal. In addition, the precoding enables such beam forming as realizes directional transmission while emphasizing desired wave for a specific user terminal.
The multiuser MIMO is realized by allocating the same resource block of a certain subframe to layers of plural user terminals. In the multiuser MIMO, the number of layers to allocate to each user is limited to one.